1. Field of the Invention
The invention relates to a directional valve with internal pressure regulator of the type having a valve housing having a longitudinal bore in which a plurality of coaxial annular ducts are recessed, the annular ducts including a pump-pressure duct and a load sensing duct; a directional valve piston which is axially displaceable in the longitudinal bore, the directional valve having an axial bore with a first radial perforation, a second radial perforation which axially spaced from the first radial perforation and a closed end; and a pressure regulator piston which is axially displaceable in the axial bore, the pressure regulator piston having a control edge, a closed end wall, an inner space with an inside diameter, and at least one radial perforation which permanently connects the inner space to the second radial perforation in the directional valve piston. A pressure regulator spring space between the closed end wall of the pressure regulator piston and the closed end of said directional valve piston, which space contains a spring which loads the closed end wall away from the closed end, is permanently connected to the load sensing duct.
2. Description of the Related Art
Such directional valves are used advantageously in mobile hydraulics for the activation of hydraulic consumers in agricultural and construction vehicles.
A directional valve of this type is disclosed in U.S. Pat. No. 6,516,614. The directional valve has an internal pressure regulator which is designed as a hollow slide. The latter has a radial perforation which is permanently connected to a radial perforation of the valve piston. Moreover, the value has an annular load-sensing duct which is permanently connected to a spring space of the pressure regulator. By virtue of the design of the directional valve, the pressure drop across a measuring diaphragm can be kept constant.
A similar directional value is also known from DE-A1-198 36 564. Here, too, a pressure regulator is arranged within the slide piston designed as a hollow piston. In order to solve the existing problem of the action of flow forces on the behavior of the directional valve, it is proposed, here, to provide on the pressure regulator piston a second control edge, by means of which an additional pressure medium flow to the working connection can be generated. By means of this compensation flow, flow forces on the pressure regulator piston and on the slide piston are to be minimized. This is also intended, in particular, to improve the response behavior of the directional valve, for example when raising and lowering operations are being initiated on power-operated lifting appliances.
The implementation of an additional control edge entails, in principle, an extra outlay in fabrication terms. The compensation of flow forces is also incomplete whenever the flows differ in their magnitude. Thus, for example, when the hydraulic consumer is a differential cylinder, the mass flow of the hydraulic medium at the two working connections A and B of the directional valve is necessarily different. In addition, such differential cylinders may themselves have different mass flow conditions in the inflow and outflow. As well as this, however, there is also a further problem: in the “neutral” position, the pressure regulator piston is to assume an unequivocal closing position. However, this is often prevented due to the fact that uncontrollable pressures build up as a result of leakages. Leakages between those annular ducts of the directional valve which have different pressures are unavoidable, and, because of fabrication tolerances, the size of these leakages cannot be foreseen. In the worst case, a movement of the consumer may consequently occur, even though this should be stationary.